1. Field of the Invention
This invention relates to a traffic offense detection machine of the type that comprises means for measuring the speed of transit of a vehicle and, connected to these, means for capturing an image of the vehicle.
2. Prior Art
Machines of this type are currently employed, in both fixed and mobile installations, for detecting speeding or other offenses on stretches of road or freeway. The speed is normally measured by a laser system using two parallel beams a known distance apart which are intersected and hence obscured by the passing vehicle. Since the distance between the beams is known, the length of time that lapses between the obscuring of the first beam and that of the second enables the speed to be calculated. Connected to the laser transducer is a control system that operates a still camera pointing in an appropriate direction to take an image of the vehicle traveling faster than the speed limit applicable to the zone where the monitoring machine is installed. The system is adjustable to enable it to be used in areas with different speed limits.
An example of a laser-type vehicle speed detector is disclosed in, for example, U.S. Pat. No. 4,902,889, the content of which should be regarded as incorporated in the present description.
Conventional systems encounter serious problems when used on multiple-lane roadways because the image-capturing machines cannot be aimed. They must therefore have a wide enough angle of view and sufficient resolution over the whole field of view to take in the entire width of the carriageway in a single shot. This is possible with a still camera but virtually impossible with a video camera. The still camera also needs a large depth of field because the delay between the instant the speed is measured and the instant when the image is taken is set at the same value irrespective of the position of the vehicle in the transverse direction of the carriageway, i.e. irrespective of the lane in which the vehicle is traveling. The delay can, if required, be calculated as a function of the measured speed, but not of the transverse position of the vehicle, which means that the image is always taken when the vehicle (whatever its speed) is within a certain zone of the carriageway. The distance between the focal plane and the vehicle license plate therefore varies depending on the transverse position of the vehicle relative to the carriageway. Hence in order to ensure that the image is always in focus the optical system of the image acquisition means must have a sufficient depth of field. This involves high costs.